Electronic device display with monitoring circuitry utilizing a crack detection resistor

ABSTRACT

An electronic device may have a flexible display such as an organic light-emitting diode display. A strain sensing resistor may be formed on a bent tail portion of the flexible display to gather strain measurements. Resistance measurement circuitry in a display driver integrated circuit may make resistance measurements on the strain sensing resistor and a temperature compensation resistor to measure strain. A crack detection line may be formed from an elongated pair of traces that are coupled at their ends to form a loop. The crack detection line may run along a peripheral edge of the flexible display. Crack detection circuitry may monitor the resistance of the crack detection line to detect cracks. The crack detection circuitry may include switches that adjust the length of the crack detection line and thereby allow resistances to be measured for different segments of the line.

This application claims the benefit of provisional patent applicationNo. 62/377,483, filed Aug. 19, 2016, and is a continuation-in-part ofpatent application Ser. No. 15/275,109, filed Sep. 23, 2016, which arehereby incorporated by reference herein in their entireties.

BACKGROUND

This relates to electronic devices, and more particularly, to electronicdevices with displays.

Electronic devices are often provided with displays. For example,cellular telephones, computers, and wristwatch devices may have displaysfor presenting images to a user.

Displays such as organic light-emitting diode displays may have flexiblesubstrates. This allows portions of the display to be bent. The tail ofa display may, for example, be bent when mounting the display in acompact device housing.

Challenges can arise in providing electronic devices with bent flexibledisplays. If care is not taken, mishandling during fabrication or stressdue to drop events may damage the display.

SUMMARY

An electronic device may have a display mounted in a housing. Thedisplay may be a flexible display such as an organic light-emittingdiode display. The display may have an array of pixels and a bent tailportion. The bent tail portion may bend about a bend axis. A displaydriver integrated circuit may supply data to columns of the pixels usingdata lines that extend across the bent tail portion. The display drivercircuit may be coupled to the bent tail portion through a flexibleprinted circuit. A gate driver circuit may supply control signals torows of the pixels using gate lines.

A strain sensing resistor may be formed on the bent tail portion of theflexible display to gather strain measurements. A temperaturecompensation resistor may be located adjacent to the strain sensingresistor. The strain sensing resistor and temperature compensationresistor may be formed from meandering metal traces. The meanderingtraces of the strain sensor may run perpendicular to the bend axis. Themeandering traces of the temperature compensation resistor may runparallel to the bend axis. Resistance measurement circuitry in thedisplay driver circuit may be used to measure the resistance of thestrain sensing and temperature compensation resistors. Strainmeasurements may be obtained by subtracting the temperature compensationresistance from the strain sensing resistance.

A crack detection line may be formed from an elongated pair of tracesthat are coupled to form a loop. The crack detection line may run alongthe peripheral edge of the flexible display. Crack detection circuitryin the display driver integrated circuit may monitor the resistance ofthe crack detection line to detect cracks. If no cracks are present,crack detection line resistance will be low. In the presence of a crack,the resistance of the crack detection line will become elevated.

A shift register in the gate driver circuit may be provided withswitches. The switches may be positioned at various positions along thelength of the crack detection line and may be selectively closed toshorten the length of the signal path in the crack detection line byvarious amounts. By closing the switches in sequence whilesimultaneously measuring the resulting resistances of the crackdetection line, the resistance of each of a plurality of segments of thecrack detection line can be determined. This allows the positions ofcracks within the crack detection line to be identified.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional side view of an illustrative electronicdevice in accordance with an embodiment.

FIG. 2 is a top view of an illustrative flexible display with straingauge monitoring resistors in accordance with an embodiment.

FIG. 3 is a top view of a portion of a flexible display with straingauge resistors in accordance with an embodiment.

FIG. 4 is a cross-sectional side view of a portion of a flexible displaywith metal traces in accordance with an embodiment.

FIGS. 5, 6, and 7 are circuit diagrams illustrative strain gaugecircuits in accordance with an embodiment.

FIG. 8 is a diagram of an illustrative display with crack detectionmonitoring circuitry in accordance with an embodiment.

FIG. 9 is a diagram of an illustrative resistance measurement circuitthat may be used to detect cracks in accordance with an embodiment.

FIG. 10 is a diagram of an illustrative display with crack detectionmonitoring circuitry in accordance with an embodiment.

FIG. 11 is a cross-sectional side view of an illustrative display withcrack detection circuitry in accordance with an embodiment.

FIG. 12A is a diagram of illustrative crack monitoring circuitry inaccordance with an embodiment.

FIGS. 12B and 12C show illustrative capacitors for the circuitry of FIG.12A in accordance with embodiments.

FIG. 13A is a diagram of an illustrative display with a crack detectionresistor having parallel paths that extend into a central strip in atail portion of the display in accordance with an embodiment.

FIG. 13B shows illustrative non-straight metal trace patterns that maybe used for the parallel lines of the crack detection resistor inaccordance with an embodiment.

DETAILED DESCRIPTION

An electronic device such as electronic device 10 of FIG. 1 may beprovided with a flexible display having monitoring circuitry. Themonitoring circuitry may include strain gauge monitoring circuitry formonitoring strain in the bent portion of a display and may includeperipheral crack monitoring circuitry. The strain gauge monitoringcircuitry may include strain gauge resistors on a bent portion of theflexible display and a strain gauge circuit that monitors for resistancechanges arising when stress is applied to the bent portion of theflexible display. The peripheral crack monitoring circuitry may have aperipheral crack detection line formed from a loop-shaped signal pathwith two parallel metal traces that runs along the periphery of theactive area of the display. A crack detection circuit may use resistancemonitoring circuitry to measure resistance changes in one or moresegments of the crack detection line that are indicative of cracking inthe line and in structures elsewhere in the display.

Electronic device 10 may be a computing device such as a laptopcomputer, a computer monitor containing an embedded computer, a tabletcomputer, a cellular telephone, a media player, or other handheld orportable electronic device, a smaller device such as a wristwatchdevice, a pendant device, a headphone or earpiece device, a deviceembedded in eyeglasses or other equipment worn on a user's head, orother wearable or miniature device, a television, a computer displaythat does not contain an embedded computer, a gaming device, anavigation device, an embedded system such as a system in whichelectronic equipment with a display is mounted in a kiosk or automobile,equipment that implements the functionality of two or more of thesedevices, or other electronic equipment. In the illustrativeconfiguration of FIG. 1, device 10 is a portable device such as awristwatch. Other configurations may be used for device 10 if desired.The example of FIG. 1 is merely illustrative.

Device 10 may have a display such as display 14. Display 14 may bemounted on the front face of device 10 in housing 12. Housing 12, whichmay sometimes be referred to as an enclosure or case, may be formed ofplastic, glass, ceramics, fiber composites, metal (e.g., stainlesssteel, aluminum, etc.), other suitable materials, or a combination ofany two or more of these materials. Housing 12 may be formed using aunibody configuration in which some or all of housing 12 is machined ormolded as a single structure or may be formed using multiple structures(e.g., an internal frame structure, one or more structures that formexterior housing surfaces, etc.). Housing 12 may have metal sidewalls orsidewalls formed from other materials,

Display 14 may be a touch screen display that incorporates a layer ofconductive capacitive touch sensor electrodes or other touch sensorcomponents (e.g., resistive touch sensor components, acoustic touchsensor components, force-based touch sensor components, light-basedtouch sensor components, etc.) or may be a display that is nottouch-sensitive. Capacitive touch screen electrodes may be formed froman array of indium tin oxide pads or other transparent conductivestructures.

Display 14 may include an array of display pixels formed from liquidcrystal display (LCD) components, an array of electrophoretic displaypixels, an array of plasma display pixels, an array of organiclight-emitting diode display pixels, an array of electrowetting displaypixels, or display pixels based on other display technologies.Configurations in which display 14 includes organic-light-emitting diodestructures may sometimes be described herein as an example.

Display 14 may have a thin flexible display layer (sometimes referred toas a pixel array, display, or flexible display) such as flexible display22. Flexible display 22 may be formed from thin-film circuitry (e.g.,thin-film transistors, thin-film organic light-emitting diodes, etc.) ona polymer substrate such as a flexible polyimide substrate. Thethin-film circuitry may be encapsulated using one or more encapsulationlayers (e.g., moisture barrier layers formed from organic and/orinorganic films). A transparent protective layer such as display coverlayer 20 may overlap flexible display 22. Cover layer 20 may be formedfrom transparent glass, clear polymer, sapphire or other crystallinematerial, ceramic, or other transparent protective layer.

Flexible display 22 may have an array of pixels 24 (pixel array 22A)that form an active area for displaying images. Flexible display 22 mayalso have an inactive tail region such as tail 22T that is free ofpixels 24. Images may be displayed for a user in pixel array 22A bypixels 24. Pixels 24 may be, for example, organic light-emitting diodepixels formed on a flexible polymer substrate (e.g., a polyimidesubstrate) and may be formed from thin-film circuitry on the substrate.

Metal traces such as metal traces 30 in flexible display 22 (e.g., datalines, control lines, etc.) may couple the circuitry of pixel array 22Awith display driver circuitry such as display driver circuitry indisplay driver integrated circuit 42. In the example of FIG. 1, circuit42 has been mounted on flexible printed circuit 32 and flexible printedcircuit 32 has been coupled to flexible display 22. With thisarrangement, display driver integrated circuit 42 may be coupled topixel array 22A using metal traces 36 in flexible printed circuit 32 andmetal traces 30 in flexible display 22. Metal traces 36 in flexibleprinted circuit 32 may be soldered to contact pads on integrated circuit42. Metal traces 36 and metal traces 30 may also form mating pads thatare coupled together at bonds 34. Bonds 34 may be anisotropic conductivefilm bonds or other conductive connections. If desired, display drivercircuitry such as display driver circuitry 42 may be coupled to pixelarray 22A with other arrangements. The use of flexible printed circuit32 to couple circuit 42 to display 22 is merely illustrative.

Flexible display 22 may have a bent portion such as bent portion 26 thatbends about bend axis 28. The inclusion of bent portion 26 in display 22may help display 22 fit within housing 12. Display driver integratedcircuit 42 may be coupled to system circuitry such as components 48 onone or more additional printed circuits such as printed circuit 46.Components 48 may include storage and processing circuitry forcontrolling the operation of device 10. Components 48 may be coupled todisplay driver circuit 42 and display 22 using connectors 45 (e.g.,board-to-board connectors).

The bending of display 22 may create stress for traces 30. If mishandledduring assembly or if subjected to stress from a drop event, there is arisk that traces 30 could become damaged. To help characterize thestresses to which display 22 is subjected, display 22 may be providedwith strain monitoring circuitry. The strain monitoring circuitry mayinclude, for example, strain gauge resistors on bent portion 26 ofdisplay 22. Crack monitoring circuitry may also be included in flexibledisplay 22 (e.g., peripheral crack detection lines may run along one ormore of the edges of pixel array 22A or other portions of display 22).

The monitoring circuitry may include resistors (strain gauge resistors,peripheral lines that have associated resistances, etc.) and circuitryfor evaluating the resistances associated with the resistors. Theresistors may be incorporated into sensitive portions of display 22(e.g., bent portion 26, the edges of pixel array 22A, etc.).

The circuitry for measuring and evaluating the resistances may be formedin display driver integrated circuit 42, in other display drivercircuitry (e.g., thin-film gate driver circuitry or gate driverintegrated circuits on the edges of pixel array 22A), or may be formedusing components 48. If desired, probe pads 38 may be formed on printedcircuit 32 and/or on display 22 and these probe pads may be contacted byprobes associated with test equipment. The test equipment may includeresistance monitoring circuitry for monitoring resistance changes instrain gauge resistors and/or crack detection line resistance changes.Test equipment may also be coupled to the circuitry of display 22 usingconnector 45 or other coupling techniques (e.g., to monitor strain gaugeresistors and/or crack detection resistors). During testing, testequipment may use electrically controlled actuators or other equipmentto automatically apply stress to display 22 (e.g., to bend display 22 inregion 26) and/or may otherwise manipulate display 22 while gatheringdata from monitoring structures in display 22. With this type of testingarrangement, the tester may, for example, direct the actuators to applyknown amounts of stress to display 22 in bent portion 26 or other regionof display 22 while using the strain gauge resistors or other monitoringsensors to gather corresponding measurements (e.g., strain gaugemeasurements). Configurations in which resistance measurement circuitryand other monitoring circuitry is incorporated into display driverintegrated circuit 42 (see, e.g., resistance measurement circuitry suchas circuit 44 in display driver integrated circuit 42 of FIG. 2) so thatstrain measurements and crack detection measurements may be made duringfabrication or during normal use of device 10 by a user may sometimes bedescribed herein as an example.

FIG. 2 is a top view of flexible display 22 in an unbent configuration.As shown in FIG. 2, pixel array 22A may include rows and columns ofpixels 24. Gate driver circuitry (e.g., thin-film gate driver circuitryrunning along the left and/or right edges of pixel array 22A) may supplyhorizontal control signals to each row of pixels 24. These horizontalcontrol signals, which may sometimes be referred to as gate linesignals, may be used to control switching transistor in the pixelcircuits associated with pixels 24 (e.g., for data loading, thresholdvoltage compensation operations, etc.). During data loading operations,data signals from display driver integrated circuit 42 may be suppliedto columns of pixels 24 via respective data lines D.

Tail portion 22T of flexible display 22 may bend around bend axis 28.Strain gauge monitoring structures such as strain gauge resistors R1 andR2 and associated strain gauge circuitry in display driver integratedcircuit 42 such as resistance measurement circuit 44 may be may be usedin monitoring strain in tail portion 22T and may form a strain gaugethat can gather real time strain gauge measurements.

The strain gauge may include one or more strain-sensing(strain-sensitive) resistors such as resistors R1. Resistors R1 maycontain meandering metal traces that change resistance when bent.Resistors R1 may be placed on tail 22T in a location that overlaps bendaxis 28, so that resistance changes in resistors R1 due to bending ofdisplay 22 in tail region 22T may be maximized.

The strain gauge may also include one or more temperature compensationstrain gauge resistors such as temperature compensation resistors R2(sometimes referred to as reference strain gauge resistors). ResistorsR2 may have meandering metal trace that match those of resistors R1 sothat both resistors R1 and resistors R2 experience the same responses tochanges in operating temperature. Resistors R2 may be placed on tail 22Tat locations that do not overlap bend axis 28 and may be oriented sothat the traces in resistors R2 run perpendicular to the traces inresistors R1. As a result, resistors R1 will change resistance when tail22T is bent about axis 28, but resistors R2 will not change resistancewhen tail 22T is bent about axis 28. This allows resistance measurementsmade with a reference resistor R2 to be subtracted from resistancemeasurements made with a strain-sensing resistor R1 to removetemperature-dependent effects from the strain gauge resistancemeasurements (e.g., to remove noise due to temperature fluctuations).

In the example of FIG. 2, display 22 has been provided with two sets ofstrain gauge resistors. A left-hand set (formed from a firststrain-sensing resistor R1 overlapping bend axis 28 and a firstassociated temperature compensation resistor R2) may be located alongthe left-hand edge of tail 22T and may measure strain along the leftside of tail 22T. A right-hand set (formed from a second strain-sensingresistors R1 overlapping bend axis 28 and a second associatedtemperature compensation resistor R2) may be located along theright-hand edge of tail 22T and may measure strain along the right sideof tail 22T. By including strain measurement circuitry along both theright and left edges of tail 22T, strain data may be gathered that issensitive to situations in which tail 22T is bent unevenly along theleft and right of tail 22T (e.g., situations in which tail 22T istwisted).

An illustrative trace layout for resistors R1 and R2 is shown in FIG. 3.As shown in FIG. 3, resistors R1 and R2 may have meandering paths formedfrom metal traces or other elongated conductive lines 50. There may beany suitable number of parallel elongated lines in each resistor (e.g.,more than 5 lines, 10-100 lines, 20-50 lines, more than 20 lines, fewerthan 200 lines, fewer than 150 lines, etc.). The width of the metaltraces forming lines 50 may be 2-10 microns, 4-8 microns, more than 3microns, less than 20 microns, or other suitable width. The length ofthe sides of each resistor may be, for example, more than 0.05 mm, morethan 0.1 mm, more than 0.5 mm, less than 1 mm, or less than 2 mm, etc.Resistors R1 and R2 may be rectangular or may have other shapes. Lines50 in resistor R1 may extend perpendicular to bend axis 28 (e.g., alongdimension Y which is aligned with the longitudinal axis of tail 22T) tomaximize bending of lines 50 and therefore changes in the resistance ofR1 when tail 22T is bent. Lines 50 in temperature compensation resistorR2 may be parallel to lines 50 in resistor R1 or may be arrangedparallel to bend axis 28 as shown in FIG. 3 to help reduce thesensitivity of resistor R2 to changes in the bending of tail 22T.

A cross-sectional side view of a portion of tail portion 22T of display22 is shown in FIG. 4. As shown in FIG. 4, tail portion 22T may have asubstrate such as substrate 52. Substrate 52 may be formed from aflexible polymer such as a layer of polyimide. Metal traces 54 may beformed on substrate 52 and may be covered with planarization layer 56.Metal traces 58 may be formed on planarization layer 56 and may becovered with planarization layer 60. In pixel array 22A, metal traces 54and 58 may be used in forming thin-film transistor structures (e.g.,source-drain terminals) and signal lines. In inactive tail portion 22Tof display 22, metal traces 54 and 58 may form control signal lines anddata lines D for carrying data from display driver integrated circuit 42to pixels 24 in pixel array 22A. Planarization layers 56 and 60 may beformed from polymers or other suitable materials. Polymer layer 62 mayserve as a neutral stress plane adjustment layer that helps move theneutral stress plane of tail 22T into alignment with traces 54 tominimize stress-induced cracking in traces 54 when tail 22T is bent.With this type of configuration, traces 58 may (as an example)experience more stress than traces 54 when tail 22T is bent.Accordingly, it may be desirable to form lines 50 for resistors R1 andR2 from the same metal layer that is used in forming lines 58 tomaximize strain gauge sensitivity. Other layers of conductive materialin display 22 may be patterned to form strain gauge resistors ifdesired. The use of the metal layer that is used in forming traces 58 toform strain gauge resistors is merely illustrative.

An illustrative strain gauge circuit is shown in FIG. 5. Resistancemeasurement circuitry 44 may be formed in display driver integratedcircuit 42 (as an example) and may be coupled to resistors R1 and R2 ontail portion 22T using metal traces 36 in flexible printed circuit 32and traces 30 in display 22. Bonds 34 between the pads formed fromtraces 36 and 30 and the portions of traces 30 and 36 that carry signalsbetween resistors R1 and R2 and circuit 44 (shown collectively as paths70) may have associated resistances Rc. For accurate strain gaugemeasurements, resistances Rc should be subtracted out of the straingauge resistance measurements. Resistance changes in resistor R1 thatare due to changes in temperature and not changes in strain can bemeasured using temperature compensation resistor R2 and can besubtracted from the measured resistance of resistor R1.

During operation, current source 64 may apply a known current I betweenterminals A and B. This causes current I to flow through resistors R1and R2, which are coupled in series between terminals A and B. Voltagesensor 66 may measure the resulting voltage V1 between terminals C and Dand voltage sensor 68 may measure the resulting voltage V2 betweenterminals D and E. The resistance of resistor R1 is equal to V1/I andthe resistance of resistor R2 is V2/I. Resistances R1 and R2 aretherefore independent of the value of resistance Rc associated withbonds 34. The resistance values for resistors R1 and R2 may bedetermined by resistance measurement circuitry (e.g., using a processorcircuit in circuitry 44) based on the known value of I and the measuredvalues of V1 and V2. The processor circuitry may also subtract R2 fromR1 to isolate changes in resistance R1 that are due to changes in thestrain on resistor R1 (e.g., bending of lines 50 about axis 28, whichcan narrow lines 50 and thereby increase the resistance of lines 50).The measured changes in resistance R1 due to strain may be used asstrain gauge measurements that reflect the amount of strain experiencedby tail portion 22T in bend region 26.

The availability of contact pads on tail portion 22T may be limited dueto the limited amount of area available on tail portion 22T. It maytherefore be desirable to couple terminals A and B to pads that arecoupled to other lines in display 22 such as lines 72. Lines 72 may be,for example, positive power supply lines (e.g., lines that carry apositive power supply voltage Vdd to pixels 24 during normal operationof display 22). By piggybacking the measurement signals for measuring R1and R2 through these contact pads, pad count can be minimized.

FIG. 6 shows how lines 72 may be omitted, if desired.

The number of pads used to measure resistances R1 and R2 may, ifdesired, be minimized using a resistance measurement arrangement of thetype shown in FIG. 7. With this arrangement, resistance measurementcircuitry 44 may measure the resistance RM1 between terminals P1 and P2and may measure the resistance RM2 between terminals P2 and P3. ResistorR1 or R2 may be coupled between terminals F and G (e.g., separatecircuits of the type shown in FIG. 7 may be used for measuring R1 andfor measurement R2). After measuring RM1 and RM2, resistance measurementcircuitry 44 can compute the value of the resistance between terminals Fand G (either R1 or R2 depending on which strain gauge resistor iscoupled between terminals F and G) by subtracting RM2 from RM1. Thiscancels out resistance Rc so that the measured strain gauge resistancevalues are independent of bond resistance.

In addition to measuring strain in display 22, display 22 mayincorporate crack detection circuitry. With one illustrativeconfiguration, which is shown in FIG. 8, a crack detection line such ascrack detection line 80 may run along some or all of the peripheral edgeof display 22. Crack detection line 80 may be formed from metal (e.g.,part of one of the metal layers used in forming pixels 24 such a gatemetal layer, source-drain metal layer, anode metal layer, cathode metallayer, etc.). Crack detection line 80 may also be formed fromsemiconductor (e.g., polysilicon or semiconducting oxide) or otherconductive material. Illustrative configurations in which crackdetection line 80 is formed from metal traces may sometimes be describedherein as an example.

Crack detection line 80 may have a loop shape formed from outgoing line80-1, end connection path 80-2, and return line 80-3 (i.e., a metaltrace that is parallel to the metal trace forming path 80-2). Thisallows line 80 to serve as a crack detection resistor. In the absence ofdamage to display 22, line 80 will be free of cracks and will becharacterized by a low resistance. In the event that display 22 issubjected to stress that forms cracks in pixels 24 or other displaycircuity, crack detection line 80, which is subjected to the samestress, will also develop cracks. The presence of cracks in crackdetection line 80 will raise the resistance of line 80. The change inthe resistance of line 80 can detected by crack detection circuitry 44in display driver circuit 42 (or external crack detection circuitry in atester, etc.). The crack detection circuitry can then report this resultto circuit components 48 (e.g., control circuitry in device 10), mayreport this result to external equipment, or may present warnings ondisplay 22 (as examples).

If desired, the crack detection circuitry for display 22 may measure theresistance of individual segments SG of line 80 such as segments SG1,SG2, . . . SGN. As shown in FIG. 8, the display driver circuitry ofdisplay 22 may include gate driver circuitry 90. Gate driver circuitry90 may receive control signals (e.g., clock signals, start and stoppulses, etc.) from display driver circuit 42 via path 92. Gate drivercircuitry 90 may contain a shift register formed from a chain ofregister circuits 84. Register circuits 84 may each supply horizontalcontrol signals (e.g., scan signals, emission enable signals, etc.) to acorresponding row of pixels 24 (e.g., signals on illustrative gate linesG). During operation, circuit 42 initiates propagation of a controlpulse through the shift register. As the control pulse propagatesthrough the shift register, each gate line G (or other set of controlsignals) is activated in sequence, allowing successive rows of pixels 24to be loaded with data from data lines D.

Gate driver circuitry 90 (e.g., some of register circuits 84) may beprovided with switches SW1, SW2, . . . SWN, each of which selectivelycreates a short between lines (parallel metal traces) 80-1 and 80-3 at adifferent respective location along the length of line 80. As thecontrol pulse propagates through the shift register, each of switchesSW1, SW2, . . . SWN is activated in sequence. As each switch is closed,resistance measurement circuitry 44 may measure the resistance of line80. When switch SW1 is closed, line 80 is shorted at switch SW1 andcircuit 44 measures the resistance of segment SG1 of line 80. Whenswitch SW2 is closed, line 80 is shorted at switch SW2 and circuit 44measures the resistance of segments SG1 and SG2 together. This processcontinues until all switches have been closed and circuit 44 measuresthe resistance of all segments of line 80 (i.e., the entire length ofline 80 from circuit 44 to connection path 80-2). Using these resistancemeasurements, the resistance of each individual segment can bedetermined by resistance measurement circuit 44. These resistancemeasurements can then be processed by the resistance measurementcircuitry to determine whether the resistance of any segment issufficiently high to reveal the presence of a crack.

Any suitable technique may be used by measurement circuitry 44 tomeasure the resistance of line 80. For example, resistance measurementcircuitry 44 may measure the resistance of line 80 by applying a knownvoltage to a capacitor of known capacitance C and discharging thatcapacitor through line 80 while incrementing a counter or otherwisetiming the decay time (RC time) associated with discharging thecapacitor. The RC time can then be used to extract a measured resistancevalue R.

Consider, as an example, a resistance measurement circuit such asillustrative resistance measurement circuitry 44 of FIG. 9. As shown inFIG. 9, resistance measurement circuitry 44 of display driver integratedcircuit 42 may be coupled to crack detection line (resistor) 80 indisplay panel 22 (see, e.g., line 80 of FIG. 8). Resistance measurementcircuitry 44 may make measurements of the resistance of line 80 whileswitches 84 (FIG. 8) are opened and closed so that segments of line 80can be monitored for the presence of cracks.

Resistance measurement circuitry 44 may have an integrator such asintegrator 100. Integrator 100 may have a capacitor such as capacitor104 and an operational amplifier such as operational amplifier 106. Theinput of integrator 100 is coupled to line 80 and can be used to receivecurrent that passes through reference resistor Rref or line 80 (ofunknown resistance R) from reference voltage source Vref.

Clock 116 may supply clock signals to control logic 112 and counter 114.The clock signals may be used to increment a count value maintained bycounter 114. When it is desired to perform a resistance measurement withintegrator 100, control logic 112 may assert a control signal thatcloses switch 102. Switch 102, which may sometimes be referred to as anintegrator reset switch, is coupled across capacitor 104 and dischargescapacitor 104 when closed. While discharging capacitor 104 to resetintegrator 100, control logic 112 may also clear counter 114.

When making resistance measurements, control logic 112 may placeresistance selection switch 108 in either a first state in which voltageVref is coupled to integrator 100 via resistor Rref or a second state inwhich voltage Vref is coupled to integrator 100 via resistor(resistance) R. In the first state, a current equal to Vref/Rref flowsinto integrator 100. In the second state, a current equal to Vref/Rflows into integrator 100, where R is the resistance of the currentlyselected segment SG of line 80 that is being measured.

During integration operations, switch 102 is placed in its open stateand the voltage on capacitor 104 rises in proportion to the currentflowing into integrator 100. The output of amplifier 106, which servesas the output of integrator 100, may be supplied to a first input ofcomparator 110. A second input of comparator 110 may be provided withreference voltage V0. Comparator 110 may compare the signals on itsfirst and second inputs and may produce corresponding output signals atits output.

When the output from integrator 100 exceeds V0, the output of comparator110 will change state (i.e., the output of comparator 110 will toggle).The change in state of the output of comparator 110 may be detected bycontrol logic 112. In response to detection of the change of state ofthe comparator output, control logic 112 can obtain the current countvalue of counter 114. This count value is proportional to the magnitudeof the current being integrated by integrator 100. The amount of timetaken to charge the integrator output to V0 (the count value of counter114) can be measured by control logic 112 in both the first state ofresistor selection switch 108 (in which current Vref/Rref flows intointegrator) and in the second state of resistor selection switch 108 (inwhich current Vref/R flows into integrator 100). Control logic 112 maythen obtain the unknown value of resistance R from the count valueobtained when switch 108 is in the first state and the count valueobtained when switch 108 is in the second state.

Strain resistor measurements (e.g., strain data from strain sensorresistor R1) and/or crack detection resistor measurements (e.g., crackdetection data such as measured resistance R from line 80) may begathered during testing and analyzed to determine whether design changesshould be made. Strain and crack detection measurements may be gatheredby a tester having test probes that are coupled to pads in display 22 orpads in flexible printed circuit 32 and/or may be gathered by a testerthat obtains digital measurements from resistance measurement circuitry44 over a digital data communications path. Strain and crackmeasurements may be gathered during manufacturing to detect damagedparts so that they can be repaired or replaced. If desired, strain andcrack data can be gathered during normal operation of device 10. Anysuitable action may be taken in response to abnormal strain or crackdata. For example, an alert may be presented on display 22 that informsa user that display 22 has been subjected to potentially damagingamounts of stress and should be serviced, historical data can begathered (e.g., to detect whether device 10 has been dropped), and/orother actions may be taken in response to gathered strain and crackdetection information. These alert techniques may also be used duringtesting and manufacturing.

As shown in FIG. 10, display 22 may be provided with two or moreconcentric rings (e.g., parallel lines formed from metal traces thatmake up a crack detection resistor) such as paths 80A and 80B. Optionalpaths forming bridging resistances RB1 may be located between paths 80Aand 80B along the left peripheral edge of display 22 and optionalbridging resistances RB2 may be located between paths 80A and 80B alongthe right peripheral edge of display 22. One or more bridgingresistances (e.g., metal trace paths) may also be formed along the upperedge of display 22.

Resistance measurement circuit M1 in resistance measurement circuitrysuch as resistance measurement circuit 44 may measure the resistancebetween terminals 122 and 124 and an optional separate measurementcircuit such as circuit M2 may be coupled between terminals 126 and 128.If desired circuit M2 may be omitted, bridging resistances RB1 and RB2may be omitted, and path 80A may be shorted to path 80B using optionalshorting paths 120. There are two concentric paths in the monitoringcircuit of FIG. 10, but additional concentric may be formed along theedge of display 22 if desired (as illustrated by dots 130).

When shorting paths 120 are present, outer peripheral crack detectionpath 80B and inner peripheral crack detection path 80A are electricallycoupled. In this type of arrangement, measurement circuit M1 may be usedto simultaneously monitor paths 80A and 80B for changes in resistance.If no cracks are present, the resistance between terminals 122 and 124will have a first resistance value. If a crack is present thatpenetrates outer path 80B but not inner path 80A, the first resistancevalue will rise to a second resistance that is larger than the firstresistance. The presence of a crack that passes through outer path 80Band inner path 80A will create a higher third resistance betweenterminals 122 and 124 (e.g., an open circuit resistance).

In arrangements in which bridging resistances such as resistances(coupling paths) RB1 and RB2 are coupled between outer path 80B andinner path 80A, crack location information can be determined frommeasured resistance information. The measured resistance between nodes122 and 124 will, for example, be different if a crack is present inpath 80B at location 132 than if the crack is present in path 80B atlocation 134 (e.g., in a different segment of path 80B). If RB1 and RB2are equal, the distance of the crack along the peripheral edge ofdisplay 22 can be determined from the resistance measurement. If RB1 andRB2 are different, the resistance network formed from paths 80A, 80B,and the paths associated with resistances RB1 and RB2 will be asymmetric(different on the left and right). As a result, the measured resistancebetween nodes 122 and 124 will correspond to a unique crack location.For example, the measured resistance will be different when a crack ispresent at location 134 (e.g., a given distance from circuit M1 alongthe left edge of display 22) than when a crack is present at location136 (e.g., the same given distance from circuit M1 along the right edgeof display 22).

When shorting paths 120 are present and circuit 44 contains onlymeasurement circuit M1 and not circuit M2, the presence of a crack thatpasses through both paths 80B and 80A will create an open circuitbetween nodes 122 and 124. As a result, it will not be possible todetermine the location of the crack along the periphery of display 22.To determine crack location in scenarios in which a crack passes throughboth outer path 80B and inner path 80A, circuit 44 can be provided withboth measurement circuits M1 and M2, as shown in FIG. 10. Paths 120 canbe omitted. With this type of arrangement, circuit M2 may be opencircuited while circuit M1 measures the resistance of the crackdetection resistor formed by coupled lines 80A and 80B. In the eventthat a crack is present that passes through both line 80B and line 80A,an open circuit resistance will be detected by circuit M1. Measurementcircuit M2 may then be placed in a short circuit state (shortingterminals 126 and 128 together) while measurement circuit M1 measuresthe resistance between terminals 126 and 128. The presence of the shortcircuit formed by circuit M2 allows peripheral path resistance to bemeasured by circuit M1 even in the presence of a crack that passesthrough both path 80B and 80A. Consider, as an example, a scenario inwhich a crack passes through path 80B in location 138 and passes throughpath 80A in location 140. During measurement with circuit M1, aloop-shaped signal path between terminals 122 and 124 will be formed.The loop-shaped path includes a first segment of path 80A betweenterminal 122 and bridge path 142, a first segment of path 80B from path142 to terminal 126, a short circuit path through circuit M2, a secondsegment of path 80B from terminal 128 to path 144, and a second segmentof path 80A from path 144 to terminal 124. The measured resistance ofthis type of path uniquely depends on the location of the crack(locations 138 and 140 in this present example) and can therefore beused to determine crack location along the edge of display 22.

If desired, crack detection circuitry for display 22 may be providedwith moisture intrusion sensitivity. A cross-sectional side view of anillustrative display with a moisture sensing configuration is shown inFIG. 11. As shown in FIG. 11, display 22 may contain an array of pixels24 in active area AA. Each pixel 24 may include a light emitting diodesuch as diode 156. Each diode 156 may be coupled to a transistor such astransistor 152 and other pixel control circuitry. Diodes such as diode156 may include emissive material 158 between cathode 154 and anode 160.

Anode 160 may be formed from a metal trace on surface 162 of thin-filmtransistor circuitry layer 150. Layer 150 may include inorganic and/ororganic dielectric layers, metal traces, one or more semiconductorlayers, and/or other layers of material for forming thin-film transistorcircuitry such as thin-film transistor 152. In peripheral portions ofdisplay 22 without pixels 24 (e.g., inactive area IA), the same layer ofmetal that is patterned to form anodes such as anode 160 (sometimesreferred to as an “anode layer”) may be used in forming peripheral paths80A and 80B. Polymer 166 (e.g., a layer of photosensitive polymerpatterned to form openings for emissive layer 158 for pixels 24) mayoverlap paths 80A and 80B. Encapsulation 164 (e.g., one or more layersof inorganic dielectric and/or organic dielectric such as polymer) mayoverlap display 22 and may help protect the structures of pixels 24 andthin-film transistor layer 150 from exposure to moisture and otherenvironmental contaminants.

In the presence of a crack along the edge of display 22, moisture mayintrude into display 22 (e.g., past encapsulation layer 164 and layer166, thereby reaching paths 80A and 80B. Paths 80A and 80B may be formedfrom metal traces that degrade and become less conductive when exposedto moisture or other environmental contaminants. For example, paths 80Aand 80B may be formed from a metal such as silver that oxidizes and/orotherwise corrodes and becomes more resistive when exposed to theenvironment (e.g., air and/or moisture). In the illustrativeconfiguration of FIG. 11, paths 80A and 80B are formed above layer 150near to encapsulation 164, so paths 80A and 80B will be sensitive todegradation in encapsulation 164. Paths 80A and/or 80B may also beformed from metal traces in thin-film transistor circuitry layer 150(e.g., metal traces patterned in a source-drain metal layer). Bymonitoring the resistance of paths 80A and/or 80B over time, thepresence of moisture in display 22 may be detected and suitable actiontaken. If desired, moisture monitoring paths such as paths 80A and 80Bmay also be used to monitor for the presence of cracks that form opencircuits, as described in connection with FIG. 10.

Another illustrative peripheral monitoring circuit arrangement is shownin FIGS. 12A, 12B, and 12C. With this type of arrangement, display 22may have measurement circuit 44 that includes a resistance measurementcircuit M1 and a capacitance measurement circuit C1. One terminal ofcircuit M1 and one terminal of circuit C1 may be coupled to outer path80B and an opposing terminal of circuit M1 and an opposing terminal ofcircuit C1 may be coupled to inner path 80A. Paths 80A and 80B may serveas first and second respective capacitor electrodes. Optional discretecapacitor structures such as capacitor C can be placed in a series oflocations along the periphery of display 22. Each capacitor C may, forexample, have interdigitated fingers as shown in FIG. 12B or may haveupper and lower overlapping electrodes as shown in FIG. 12C (e.g.,electrodes that overlap in a direction perpendicular to the surface ofthe display). Dielectric (e.g., polymer 166 and/or polymer and/orinorganic dielectric in layer 150 of FIG. 11) may be interposed betweenpaths 80A and 80B. When moisture is present, the moisture will alter thedielectric constant of the dielectric between paths 80A and 80B. Theresulting change in capacitance between paths 80A and 80B can bemonitored by circuit C1 and used to detect moisture intrusion. Moisturemay also alter the resistance through the dielectric from path 80B to80A, which can be monitored using circuit M1.

In addition to or instead of using circuits M1 and C1 to measure fordegradation due to environmental contaminants, measurements fromcircuits M1 and C1 can be used to detect cracks. Consider, as anexample, a scenario in which a crack is present in path 80B at location172. This crack will electrically isolate portion 170 of path 80B fromthe rest of path 80B. As a result, the resistance measured by circuit M1between paths 80B and 80A through the dielectric separating paths 80Band 80A will rise in proportion to the shortened length of path 80B. Forexample, if the length of path 80B is cut in half by the presence of acrack, the measured resistance will double. At the same time, themeasured capacitance between paths 80B and 80A will increase as thelength of path 80B is reduced (e.g., the capacitance measured by circuitC1 will decrease in proportion to the decrease in length of path 80B).Measured resistance and/or capacitance can therefore be used todetermine the location of the crack.

As shown in FIG. 13A, paths 80A and 80B may, if desired, be locatedcentrally within tail portion 22T of display 22. For example, in ascenario in which tail 22T has a total width W2, paths 80A and 80B maybe located within a central strip of width W1, where W1 is less than 50%of W2, less than 20% of W2, is less than 10% of W2, or is anothersuitable fraction of W2. The traces that form paths 80A and 80B in tail22T may be straight or may have a non-straight shape such as aserpentine shape or other meandering path shape to help resist crackingwhen tail 22T is bent. For example, paths 80A and 80B may be formed fromchains with circular links, elliptical links, butterfly patterns, singleor double serpentine path shapes and/or other crack-resistant shapes.These path patterns may also be used in forming data lines D. FIG. 13Bis a diagram showing illustrative chain-shaped meandering metal tracesfor forming paths 80B and 80A in tail 22T. Other non-straight lineshapes may be used for the metal traces forming paths 80B and 80A, ifdesired.

By placing paths 80B and 80A in the center of tail 22T as shown in FIG.13A, paths 80B and 80A will be sensitive to damage to display 22 that iscaused by dropping device 10 on its lower edge. Corner impacts can bedetected using strain gauges located on either side of paths 80A and 80B(see, e.g., resistors R1 and R2 of FIG. 2). The strain gauges may,however, exhibit reduced sensitivity to lower edge impacts. Potentialdamage to display 22 from lower edge impacts can, however, be measuredby using circuitry 44 to monitor paths such as paths 80B and 80A in thecenter of tail 22T, as shown in FIG. 13A.

Paths 80A and 80B may all be formed in a first source-drain metal layerin thin-film transistor circuitry layer 150 of FIG. 11, may all beformed in a different second source-drain metal layer in layer 150, ormay include two lines formed in the first source-drain metal layer andtwo lines formed in the second source-drain metal layer (as examples).Tail 22T may include a polymer coating layer (sometimes referred to as aneutral stress plane adjustment layer) that helps place a neutral stressplane of tail 22T close to the metal traces on tail 22T to reducestress-induced cracks. For example, the neutral stress plane of tail 22Tmay be located within a substrate layer in tail 22T. The firstsource-drain metal layer may be located between the substrate of tail22T and the neutral stress plane adjustment layer. The secondsource-drain metal layer may be interposed between the firstsource-drain metal layer and the neutral stress plane adjustment layer.With this type of arrangement, the second source-drain metal layer maybe located farther from the neutral stress plane in the substrate oftail 22T than the first source-drain metal layer. As a result, placingall four of the lines for paths 80A and 80B in tail 22T in the secondsource-drain metal layer may make paths 80A and 80B more sensitive tocracks in the bend in tail 22T than placing all four of these lines inthe first source-drain metal layer.

The foregoing is merely illustrative and various modifications can bemade to the described embodiments. The foregoing embodiments may beimplemented individually or in any combination.

What is claimed is:
 1. A display system, comprising: a flexible displayhaving pixels; a crack detection resistor that runs along a peripheraledge of the flexible display, wherein the crack detection resistor hasparallel first and second lines along the peripheral edge and has aplurality of bridging resistances located at respective locations alongthe peripheral edge of the flexible display, wherein the crack detectionresistor has first and second ends with first and second respectiveterminals, wherein each of the bridging resistances is coupled betweenthe first line and the second line, wherein each of a first plurality ofthe bridging resistances has a first resistance and is coupled betweenthe first and second lines along a first side of the pixels and whereineach of a second plurality of the bridging resistances is coupledbetween the first and second lines along a second side of the pixels andhas a second resistance that is different than the first resistance; andmeasurement circuitry coupled to the flexible display that is configuredto make measurements on the crack detection resistor to determine alocation of a crack in the first and second lines of the crack detectionresistor based on the first and second resistances and a change in totalresistance between the first and second terminals.
 2. The display systemdefined in claim 1 wherein the first and second lines comprise a metalthat corrodes upon exposure to moisture.
 3. The display system definedin claim 1 wherein the first and second lines comprise silver.
 4. Thedisplay system defined in claim 1 wherein the pixels comprise anodesformed from a layer of anode metal and wherein the first and secondlines comprise portions of the layer of anode metal.
 5. The displaysystem defined in claim 1 wherein the pixels are formed from thin-filmtransistor circuitry and wherein the first and second lines includelines formed from a source-drain metal layer in the thin-film transistorcircuitry.
 6. A display system, comprising: a flexible display havingpixels and a bent tail portion that extends away from the pixels; acrack detection resistor that runs along a peripheral edge of theflexible display and that extends into a central strip in the bent tailportion, wherein the crack detection resistor includes a first line witha first pair of terminals and a second line with a second pair ofterminals; first and second pluralities of data lines for the flexibledisplay, wherein the central strip is interposed between the first andsecond pluralities of data lines; and resistance measurement circuitryconfigured to make a first resistance measurement between the first pairof terminals to detect a presence of a crack in the first and secondlines and to form a short circuit between the second pair of terminalsand make a second resistance measurement to determine a location of thecrack in response to detecting the crack.
 7. The display system definedin claim 6 wherein the first and second lines are parallel and arecoupled by a series of bridging paths.
 8. The display system defined inclaim 6 further comprising a display driver integrated circuit thatsupplies data to columns of the pixels over the first and secondpluralities of data lines.
 9. The display system defined in claim 8wherein the resistance measurement circuitry is in the display driverintegrated circuit.
 10. The display system defined in claim 6 whereinthe first and second lines have straight portions that run along theperipheral edge and have non-straight portions in the bent tail portion.11. The display system defined in claim 10 wherein the bent tail portionhas a first width, wherein the central strip has a second width, andwherein the second width is less than 20% of the first width.
 12. Thedisplay system defined in claim 6 wherein the pixels are formed fromthin-film transistor circuitry including at least first and second metallayers and wherein the crack detection resistor has first and secondparallel lines in the central strip that are formed from the first metallayer.
 13. The display system defined in claim 6 wherein the pixels areformed from thin-film transistor circuitry including at least first andsecond metal layers and wherein the crack detection resistor has firstand second lines in the central strip that are formed from portions ofthe first metal layer and from portions of the second metal layer.
 14. Adisplay system, comprising: a display having pixels; first and secondconcentric paths that run along a peripheral edge of the display; and afirst resistance measurement circuit coupled to the first path and asecond resistance measurement circuit coupled to the second path,wherein the first resistance measurement circuit is configured to make afirst measurement to detect a presence of a crack in both of the firstand second concentric paths when the second resistance measurementcircuit is in an open circuit state, wherein the second resistancemeasurement circuit is decoupled from the second path in the opencircuit state, and wherein the first resistance measurement circuit isconfigured to make a second measurement when the second resistancemeasurement circuit is in a closed circuit state to determine a locationof the crack in response to detecting the presence of the crack.
 15. Thedisplay system defined in claim 14, further comprising: resistive pathsthat couple the first path to the second path, wherein each resistivepath couples the first path to the second path at a different respectivelocation along the first and second paths.
 16. The display systemdefined in claim 15 wherein the first resistance measurement circuitmeasures the resistance of the first path to determine a location of acrack in the first path and the second resistance measurement circuitmeasures the resistance of the second path to determine a location of acrack in the second path.
 17. The display system defined in claim 14wherein the first path is interposed between the second path and thepixels.
 18. The display system defined in claim 17 further comprising: athird concentric path that runs along the peripheral edge of thedisplay, wherein the second path is interposed between the first pathand the third path.